Adjustable bone screw assembly

ABSTRACT

A bone screw assembly includes a screw body, including anchor portion and rod-receiving portion, and a rod seat movably mounted in the screw body to allow for controlled angulation between a spinal connection element disposed in the rod seat and the screw body. The rod seat is pivotable in one or more selected directions about one or more axes relative to the screw body. The rod seat may include a first lower rod seat element disposed in a recess of the screw body having a cylindrical bottom surface to facilitate pivoting in a first direction. A second lower rod seat element is stacked on the first lower rod seat element and has a conical bottom surface abutting a top surface of the first lower rod seat element to facilitate pivoting in a second direction.

FIELD OF THE INVENTION

The present invention relates to spinal connection devices used inorthopedic surgery. More particularly, the present invention relates toa bone screw for coupling a spinal rod to a bone, such as the pedicle.

BACKGROUND OF THE INVENTION

Spinal fixation systems may be used in surgery to align, adjust and/orfix portions of the spinal column, i.e., vertebrae, in a desired spatialrelationship relative to each other. Many spinal fixation systems employa spinal rod for supporting the spine and for properly positioningcomponents of the spine for various treatment purposes. Vertebralanchors, comprising pins, bolts, screws, and hooks, engage the vertebraeand connect the supporting rod to different vertebrae. The size, lengthand shape of the cylindrical rod depend on the size, number and positionof the vertebrae to be held in a desired spatial relationship relativeto each other by the apparatus.

Spinal fixation elements can be anchored to specific portions of thevertebra. Since each vertebra varies in shape and size, a variety ofanchoring devices have been developed to facilitate engagement of aparticular portion of the bone. Pedicle screw assemblies, for example,have a shape and size that is configured to engage pedicle bone. Suchscrews typically include a threaded shank that is adapted to be threadedinto a vertebra, and a head portion having a spinal fixationelement-receiving portion, which, in spinal rod applications, is usuallyin the form of a U-shaped slot formed in the head portion for receivingthe rod. A set-screw, plug, cap or similar type of closure mechanism isused to lock the rod into the rod-receiving portion of the pediclescrew. In use, the shank portion of each screw is then threaded into avertebra, and once properly positioned, a spinal fixation rod is seatedthrough the rod-receiving portion of each screw. The rod is locked intoplace by tightening a cap or similar type of closure mechanism tosecurely interconnect each screw and the fixation rod. Other anchoringdevices also include hooks and other types of bone screws.

Monoaxial screws are a type of screw in which the longitudinal axis ofthe threaded shank is fixed relative to the head portion, or rod slot.The longitudinal axis of the threaded shank may be aligned with thelongitudinal axis of the head portion, and/or the threaded shank extendsat a fixed angle relative to the head. In fixed pedicle screws, whichare used in the pedicle region of the vertebra, the threaded shank isrigidly connected to or integrally formed with the head such that theorientation of the threaded shank is fixed with respect to the head.

Polyaxial pedicle screws have been designed to allow angulation of oneportion of the screw relative to another portion of the screw and thespinal fixation element coupled to one portion of the screw. Forexample, polyaxial pedicle screws allow for a shaft portion to pivotrelative to a rod-receiving portion in all directions about a 360° arcaround the rod-receiving portion. Polyaxial screws may be useful forpositioning bone anchors on adjacent vertebrae, when the close proximityof adjacent vertebrae can result in interference between the boneanchors. Polyaxial screws allow for pivoting of the screws in anydirection out of alignment with each other to avoid such interference.

An example of such a polyaxial pedicle screw assembly is described indetail in U.S. Patent Application Publication Number US 2004/0186473entitled “Spinal Fixation Devices of Improved Strength and Rigidity”,U.S. Patent Application Publication Number US 2004/0181224 entitled“Anchoring Element for Use in Spine or Bone Surgery, Methods for Use andProduction Thereof” and U.S. Patent Application Publication Number US2003/0100896, entitled “Element With a Shank and a Holding ElementConnected to It for Connecting to a Rod”, the contents of which areherein incorporated by reference.

Polyaxial and multi-axial screws, which allow the screw shank to pivotin all directions about the head portion, can be difficult to controland often result in movement of the screw shank in planes in whichmovement is not desirable. For example, during vertebral body rotationmaneuvers, which require application of force to the screw head, it isnot desirable for the screw shank to move relative to the screw head.

In addition, prior art bone screw systems are not optimized fornon-fusion systems, which may employ flexible rods. Without the supportof fused bones, non-fusion systems must withstand the applied load for alonger time, even for the lifetime of a patient.

An additional complication of non-fusion stabilization systems is aloosening between the screw and bone. Monoaxial and polyaxial screws ofthe prior art tend to have a relatively large distance between thecenter of the rod and the vertebral body surface, resulting in arelatively large moment applied to the screw body by the rod when thescrew is implanted, leading to loosening and/or breakage.

SUMMARY OF THE INVENTION

The present invention provides an adjustable bone screw assembly thatprovides for controlled adjustment of a spinal connection element, suchas a spinal rod, received in a body of the bone screw assembly relativeto the body of the bone screw. The adjustable bone screw assembly mayallow the spinal connection element received in a receiving portion ofthe assembly to pivot in at least two directions about two differentaxes, while limiting movement in other selected directions. In addition,the bone screw assembly may reduce a distance between a center of a rodconnected to the bone screw assembly and the vertebral bone surfacesecured by the bone screw assembly. The adjustable bone screw assemblythus allows for sufficient adjustability for rod placement andorientation, while reducing the moment applied on the screw.

According to a first aspect of the invention, a bone anchor assemblycomprises a bone anchor having a distal shaft extending along alongitudinal axis configured to engage bone, a proximal head portion anda rod seat disposed within the proximal head portion for seating thespinal rod, wherein the rod seat allows for a controlled pivotingmovement of the spinal rod about two selected axes in two directionsrelative to the head portion.

According to another aspect of the invention, a bone anchor assemblycomprises a bone anchor having a distal shaft extending along alongitudinal axis configured to engage bone and a proximal head, a firstlower rod seat element disposed in a recess of the proximal head andhaving a cylindrical bottom surface and a second lower rod seat elementstacked on the first lower rod seat element. The second lower rod seatelement has a conical bottom surface abutting a top surface of the firstlower rod seat element and an upper surface for seating the spinal rod.

A retaining means may movably retain the first lower rod seat element inthe recess. A retaining means may rotatably retain the second lower rodseat element on the first lower rod seat element.

According to another aspect of the invention, a bone anchor assemblycomprises a bone anchor having a distal shaft extending along alongitudinal axis configured to engage bone and a proximal head and arod seat disposed in the proximal head for seating a spinal rod. The rodseat includes a top surface for receiving the spinal rod and a conicalbottom surface disposed in a corresponding recess to allow pivoting ofthe spinal rod about a longitudinal axis of the conical bottom surface.

According to another aspect of the invention, a bone anchor assembly,comprises a bone anchor portion having a distal shaft extending along alongitudinal axis configured to engage bone and a head portion fixed toa proximal end of the bone anchor portion. The head portion isconfigured to receive a spinal rod and allow for a controlled pivotingmovement of the spinal rod about two selected axes in two directionsrelative to the head portion.

According to another aspect of the invention, a method of connecting twovertebrae is provided. The method comprises the steps of inserting afirst portion of a spinal rod in a first rod-receiving portion of afirst bone screw assembly connected to a first vertebra, adjusting anorientation of the spinal rod relative to the first rod-receivingportion in a first direction and adjusting an orientation of the spinalrod relative to the first rod-receiving portion in a second direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following description and apparentfrom the accompanying drawings, in which like reference characters referto the same parts throughout the different views. The drawingsillustrate principles of the invention and, although not to scale, showrelative dimensions.

FIGS. 1A and 1B illustrate an assembled bone screw assembly including aspinal rod movably received therein according to an illustrativeembodiment of the invention.

FIG. 2 is a top view of the assembled bone screw assembly, illustratinga first degree of freedom.

FIG. 3 is a side view of the assembled bone screw assembly, illustratinga second degree of freedom.

FIGS. 4A and 4B illustrate possible ranges of motion of the spinal rodrelative to the bone screw body according to different embodiments ofthe invention.

FIGS. 5A and 5B are exploded views of a biaxial bone screw assemblyaccording to one embodiment of the invention.

FIGS. 6A and 6B illustrate a biaxial bone screw assembly according toanother embodiment of the invention.

FIG. 7 illustrates a biaxial bone screw assembly of an embodiment of theinvention including a retaining pin for coupling the first lower rodseat element and the second lower rod seat element.

FIGS. 8A-8B illustrates an embodiment of the first lower rod seatelement of the biaxial bone screw assembly of FIG. 7.

FIG. 9A-9C illustrates an embodiment of the second lower rod seatelement of the biaxial bone screw assembly of FIG. 7.

FIG. 10 illustrates the retaining pin employed by the biaxial bone screwassembly of FIG. 7.

FIGS. 11A and 11B illustrate an adjustable bone screw assembly accordingto another embodiment of the invention.

FIG. 12A illustrates one embodiment of a set screw used in a biaxialbone screw assembly of an illustrative embodiment of the invention.

FIG. 12B illustrates another embodiment of a set screw used in a biaxialbone screw assembly of an illustrative embodiment of the invention.

FIG. 13 illustrates a biaxial bone screw assembly of an illustrativeembodiment of the invention including a tapered anchor portion.

FIG. 14 illustrates a biaxial bone screw assembly of an illustrativeembodiment of the invention, a polyaxial bone screw assembly of theprior art and a monoaxial bone screw assembly of the prior art,comparing a rod-to-bone distance between the three bone screwassemblies.

FIG. 15 illustrates biaxial bone screw assembly of an illustrativeembodiment of the invention and a polyaxial bone screw assembly of theprior art when the spinal rods are rotated by 20 degrees relative to therespective anchor portions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved bone screw assembly in aspinal connection system. One skilled in the art will recognize that theinvention is not limited to use in bone or in spinal surgery, and thatthe instrument and methods described herein can be adapted for use withany suitable surgical device to be moved into a selected position in avariety of medical procedures. The present invention will be describedbelow relative to certain exemplary embodiments to provide an overallunderstanding of the principles of the structure, function, manufacture,and use of the instruments disclosed herein. Those skilled in the artwill appreciate that the present invention may be implemented in anumber of different applications and embodiments and is not specificallylimited in its application to the particular embodiments depictedherein.

During spinal deformity surgeries, it may be necessary to de-rotate thevertebral bodies to normalize the spine. Due to varying patient anatomy,insertion of fixed angle screws, where the anchor portion of the screwextends at a fixed angle relative to the rod-receiving portion of thescrew can be difficult. Polyaxial and multi-axial screws, which allowthe screw shank to pivot in all directions about the rod-receiving headportion, can be difficult to control and often result in undesirablemovement in certain planes. An adjustable bone screw assembly allows forangulation of a spinal rod or other implant relative to the body of thescrew that receives the spinal rod or other implant therein. Forexample, a biaxial bone screw assembly, different embodiments of whichare illustrated in FIGS. 1-10 and 12-15, allows for angulation of aspinal rod or other implant relative to the body of the screw in atleast two planes, while minimizing a distance between a rod center and asurface of the bone secured by the biaxial bone screw assembly. As shownin FIGS. 11A and 11B, an adjustable bone screw assembly may be uniaxialand permit movement about a single selected axis. The adjustable bonescrew assembly of the invention is not limited to a uniaxial or biaxialmovement and may allow adjustment of a spinal rod or implant relative toa screw body that retains the rod or implant in any selected number ofdirections and about any selected number of axes.

The adjustable bone screw assembly of the present invention may allow asurgeon to rotate vertebral bodies and facilitates rod placement intothe rod-receiving portion. The adjustable bone screw assembly allows fora surgeon to achieve an ideal orientation of the spinal rod relative tothe bone screw, without requiring the spinal rod to have apredetermined, fixed orientation, or necessarily be perpendicular to thelongitudinal axis of the screw shank.

The exemplary adjustable bone screw assemblies of the illustrativeembodiments of the invention may be employed to engage one or morespinal connection elements to bone. For example, a bone screw assemblymay be employed to fix a spinal plate, rod, and/or cable to a vertebraof the spine. Although the exemplary bone screw assemblies describedbelow are designed primarily for use in spinal applications, andspecifically the pedicle region of a vertebra, one skilled in the artwill appreciate that the structure, features and principles of theexemplary bone screw assemblies, as well as the other exemplaryembodiments described below, may be employed to couple any type oforthopedic implant to any type of bone or tissue. The bone screwassembly described herein facilitates the correction of the position,for example, the angular orientation, of the vertebra in which the bonescrew is implanted, as described in U.S. patent application Ser. No.11/073,352 entitled “Instruments and Methods for Manipulating aVertebra”, filed on Mar. 4, 2005 and U.S. patent application Ser. No.11/073,325 file on Mar. 4, 2005, the contents of which are hereinincorporated by reference.

The illustrative adjustable bone screw assembly may be used to attach anon-rigid member to bone. For example, the adjustable bone screwassembly may be used to attach a rod, ligament, bar, cable or othernon-rigid member extending between and connecting two bone screws, forexample for connecting superior and inferior vertebra. Alternatively,the bone screw assembly may be used to attach a rigid member to bone.While the invention will be described with respect to a bone screwassembly that receives a spinal rod that is movably about two axesrelative to the bone screw assembly, the invention is not limited tospinal rods and may be used with any suitable spinal connection elementto be coupled to bone.

According to one aspect of the invention, a biaxial bone screw assembly10, an embodiment of which is shown in FIGS. 1A and 1B, is provided toallow movement of a spinal rod in two directions relative to a bonescrew body. The biaxial bone screw assembly 10 has a screw body 11,which includes a distal anchor portion 13 for anchoring the screwassembly to bone and a proximal head portion 14. The proximal headportion 14 may form a receiving portion, such as a rod-receivingportion, for receiving a spinal rod 12 or other spinal connectionelement. An adjustable rod seat 18, disposed within the head portion 14,seats the spinal rod 12, thereby coupling the spinal rod 12 to a boneanchored by the anchor portion 13. The movable rod seat 18 mayalternatively be sized and configured to receive another spinalconnection element in an adjustable position.

The orientation of the rod seat 18 may be selectively adjusted tocontrollably adjust the orientation of the spinal rod 12 relative to thebody 11 of the bone screw assembly 10. Preferably, the illustrative rodseat 18 allows for pivoting of the spinal rod 12 about two separate axesin at least two directions relative to the body 11 of the screwassembly, though one skilled in the art will recognize that the rod seat18 and spinal rod 12 of the invention may alternatively pivot about oneor several axes. For example, the illustrative rod seat 18 allows thespinal rod 12 to pivot about a first axis X-X and/or a second axis Y-Yrelative to the anchor portion 13 and/or head portion 14 to provide twodegrees of freedom of the spinal rod position relative to the bone screwbody 11.

The rod seat 18 extends along an axis R-R, which defines and correspondsto the longitudinal axis of the spinal rod 12. In a default position,the axis R-R is preferably perpendicular to the longitudinal axis S-S ofthe body 11, though one skilled in the art will recognize that the rodseat 18 may have any suitable default orientation.

As shown in FIG. 2, the spinal rod 12 may pivot about axis X-X in adirection indicated by arrow 20. In the illustrative embodiment, theaxis X-X extends through the center of the rod seat 18, perpendicular tothe axis R-R and aligns with the longitudinal axis S-S of the screw body11 when the rod seat 18 is in a default position, so that the spinal rod12 sweeps through a first plane P1 shown in FIG. 1A. In this manner, theillustrative spinal rod 12 spins about the screw body 11. Theillustrative first plane P1 is substantially parallel to the coronalplane of the body when the bone screw assembly 10 is employed in apatient. However, one skilled in the art will recognize that the firstaxis X-X about which the spinal rod 12 can pivot may have any suitableorientation and is not limited to the illustrative orientation.

As shown in FIG. 3, the spinal rod 12 may also or alternatively pivot ina second plane P2 about axis Y-Y in a direction indicated by arrow 30.In the illustrative embodiment, the axis Y-Y extends substantiallyperpendicular to the axis X-X, the axis S-S, and the axis R-R of thespinal rod 12 when the rod seat 18 is in a default position, though oneskilled in the art will recognize that the second axis about which thespinal rod 12 can pivot may have any suitable orientation. The plane P2corresponds to the sagittal plane in a body when the illustrativebiaxial bone screw assembly is implanted in the body. As shown, theadjustability about axis Y-Y allows for one end 121 of the spinal rod 12to pivot towards the body while the other end of the spinal rod 122pivots away from the body. However, one skilled in the art willrecognize that the second axis Y-Y about which the spinal rod 12 canpivot may have any suitable orientation and is not limited to theillustrative orientation.

The adjustment about the two axes X-X and Y-Y can be mutually exclusive,or concurrent. A user may rotate the spinal rod 12 by a selected amountabout only the X-X axis, only the Y-Y axis, or both. The rotation aboutone axis may cause the orientation of the other axis to shift, or theorientation of each axis X-X and Y-Y may be fixed independently of theorientation of the other axis. For example, as shown in FIG. 2, therotation of the spinal rod about axis X-X may cause the axis Y-Y toshift by the amount of rotation of the spinal rod to position Y′-Y′, sothat the axis Y-Y remains parallel to the axis R-R. As shown in FIG. 3,the rotation of the rod 12 about axis Y-Y may cause the axis X-X toshift to the position X′-X′ shown in FIG. 3. Alternatively, one or bothof the axes X-X and/or Y-Y may be fixed regardless of the rotation ofthe spinal rod 12 about the other axis.

FIGS. 4A and 4B illustrate possible areas of movement of the spinal rodrelative to the bone screw body according to different embodiments ofthe invention. As shown in FIG. 4A, the spinal rod 12 may pivot througha cone of motion 141 defining the possible orientations of the spinalrod 12 relative to the body 11 through rotation about the X-X and/or Y-Yaxis. Alternatively, as shown in FIG. 4B, the spinal rod 12 may beconfined to paths 142, 143, defined by axes X-X and Y-Y, respectively.

In addition, for cylindrical rods or other spinal connection elements,the rod 12 may be rotated about axis R-R and/or slide within the rodseat 18, providing a third degree of freedom for attaining a selectedorientation of the spinal rod relative to the screw assembly 120.

FIGS. 5A and 5B are exploded views of a biaxial bone screw assembly 10,illustrating the individual components of the assembly 10 thatfacilitate the biaxial adjustability according to an illustrativeembodiment of the invention.

The bone anchor 13 comprises a distal shaft 118 configured to engagebone. The distal shaft 118 of the bone anchor 13 has a shaft diameter120 and a longitudinal axis S-S. The distal shaft 118 may include one ormore bone engagement mechanisms to facilitate gripping engagement of thebone anchor to bone. In the illustrated embodiment, the distal shaft 118includes an external thread 124 extending along at least a portion ofthe shaft for engaging bone. In the illustrated embodiment, the externalthread 124 is a single lead thread that extends from a distal tip 126 ofthe shaft to the head portion 14, though one skilled in the art willrecognize that the external thread may extend along any selected portionof the shaft and have any suitable number of leads. Other suitable boneengagement mechanisms include, but are not limited to, one or moreannular ridges, multiple threads, dual lead threads, variable pitchedthreads and/or any conventional bone engagement mechanism.

The head portion 14 is sized and configured to receive a spinal rod orother suitable spinal connection element. The illustrative head portion14 forms a U-shaped slot 142 or other suitable opening at the distal endof the shaft 118 for receiving a spinal rod and to couple the spinal rodto the bone anchor portion 13. The illustrative head portion 14 may besubstantially similar to a head portion of a monoaxial or polyaxialscrew assembly of the prior art. While the illustrative head portion 14is sized and shaped to receive a spinal rod, one skilled in the art willrecognize that the head portion 14 may be configured to accommodate anysuitable spinal connection element.

Alternatively, the head portion 14 of the screw assembly can be closedat the proximal end with an opening to receive a set screw. The closedhead has a side opening for receiving a spinal rod. The distal end ofthe side opening also has features similar to that of a U-shaped head.

As shown, the illustrative head portion 14 for receiving a rod isrigidly coupled to or integral with the anchor portion 13 to form thescrew body 11, though one skilled in the art will recognize that thehead portion 14 may alternatively be movably coupled to the anchorportion 13 to provide additional adjustability.

The longitudinal axis S-S of the bone anchor portion 13 preferablyaligns with a longitudinal axis extending through the head portion 14.However, one skilled in the art will recognize that the head portion 14may alternatively be offset from or extend at a selected angle relativeto the anchor portion 13.

In other embodiments, a spinal connection element may be coupled to thebone anchor portion 13 by alternative coupling mechanisms in place ofthe illustrative head portion 14. For example, the receiving portion mayalternatively comprise an offset coupling mechanism, such as a bandclamp, sacral extender, or a lateral off-set connector.

The illustrative head portion 14 is also configured to movably mount therod seat 18, which directly receives the rod 12. As described above, therod seat 18 allows for pivoting of the rod 12 in two selected directionsabout a first axis X-X and/or a second axis Y-Y. The rod seat 18 of theembodiment of FIGS. 5A and 5B comprises three cooperating elements forseating the rod 12 and allowing movement of the rod relative to thescrew body 11. The head portion 14 includes a recess 144 formed in thebottom of the U-shaped slot 142 for movably receiving the rod seat 18.The recess 144 preferably has a concave, cylindrical shape to allowpivoting of the rod seat 18 within the recess 144 in a first direction.

As shown, the rod seat 18 comprises a first lower element 180 a disposedin the recess 142, a second lower element 180 b stacked on the firstlower element 180 a and an upper element 181. The first lower element180 a and second lower element 180 b cooperate to define a bottomportion of the rod seat 18 that allows rotation of the rod 12 about twoseparate axes. The rod 12 is received between the second lower element180 b and the upper element 181. A closure mechanism, illustrated as aset screw 170, secures the spinal rod 12 or other suitably configuredspinal connection element within the slot 142 of the head portion 14 andlocks the rod 12 and rod seat 18 in the selected orientation within andrelative to the screw body 11.

The first lower element 180 a facilitates pivoting of the rod 12 aboutone axis, which is axis Y-Y in the illustrative embodiment. Theillustrative first lower element 180 a is substantially half-tubular orcylindrical in shape and includes a cylindrical bottom surface 1801configured to be received in and mate with a concave surface of therecess 144. The bottom surface is not limited to a cylindrical shape andmay have any shape that is generated by one or more sweeping linesegments, which may include one or more curved or straight lines, aboutthe axis Y-Y, thus allowing rotation about the axis Y-Y. Another exampleis a curved surface with a grooved channel.

The recess 144 preferably has a shape that matches the bottom surface1801 and allows sliding/rotation of the first lower element 180 arelative to the screw body about axis Y-Y, which corresponds to thelongitudinal axis the cylindrical body forming the first lower element180 a.

The side surfaces 1803, 1804 of the first lower element 180 a may besubstantially flat and abut side surfaces of the recess 144 to confinemovement of the first lower element 180 a to rotation about a singleaxis Y-Y. Alternatively, the side surfaces 1803, 1804 may have anysuitable size, shape, configuration and/or orientation to facilitatecontrolled movement of the first lower element 180 a and thus the spinalrod 12 relative to the screw body 11.

The upper surface 1802 of the first lower element 180 a is sized andconfigured to receive the second lower element 180 b and allow rotationof the second lower element 180 b in a second direction relative to thefirst lower element 180 a and the body 11 of the screw assembly 10.

The second lower element 180 b has an axisymmetric mating surface thatmates with the first lower element 180 a to provide another rotationaldegree of freedom. The axisymmetric surface is symmetric about a centralaxis, allowing the second lower element to pivot about the central axis.The illustrative second lower element 180 b includes a substantiallyconical-shaped bottom mating surface 1807 that is stacked on a similarlyshaped recess on the upper surface 1802 of the corresponding first lowerelement 180 a. As shown, the second lower element 180 b has taperedouter walls 1807 to allow rotation of the second lower element aboutaxis X-X. The bottom surface 1807 of the second lower element 180 b andcorresponding recess in the upper surface 1802 of the first lowerelement 180 a are preferably symmetrical about axis X-X to facilitatecontrolled spinning of the second lower element 180 b within the firstlower element 180 a. Alternatively, the axisymmetric bottom surface 1807can be spherical, cylindrical, or have another suitable shape tofacilitate rotation of the second lower element 180 b relative to thefirst lower element 180 a about an axis, such as the illustrative X-Xaxis.

The second lower element 180 b further includes a substantiallycylindrical upper surface 1808 for receiving the spinal rod. The uppersurface 1808 preferably matches the profile of the spinal rod 12 toallow seating of the spinal rod thereon.

According to another embodiment of the invention, shown in FIGS. 6A and6B, the position or shape of the first lower element 180 a and thesecond lower element 180 b can be exchanged. For example, in the biaxialbone screw assembly 10′ of FIGS. 6A and 6B, the first lower element 180a′ has a conical shape to facilitate rotation about axis S-S. The secondlower element 180 b′ stacked on the first lower element 180 a′ has acylindrical shape to facilitate rotation about axis X-X. Preferably, thebottom surface 1801′ of the first lower element 180 a′ and the opposingdistal end slot 142 in the screw head have mating conical shapes thatallows rotation about Axis S-S. In addition, the top surface 1802′ ofthe first lower element 180 a′ and opposing bottom surface 1807′ of thesecond lower element 180 b′ have mating cylindrical shape that allowsrotation about X-X.

In the illustrative embodiment, the top element 181 of the spinal rodhas a flat bottom surface 1812 and a curved, substantially sphericalupper surface 1814 to allow for pivoting of the rod seat 18 about bothaxes X-X and Y-Y. Alternatively, the bottom surface 1812 may becylindrical. One skilled in the art will recognize that the top element181 may have any suitable size, shape and configuration.

Alternatively, the top element 181 may comprise two separate elements,including an axisymmetrical mating surface and a cylindrical matingsurface to facilitate adjustment of the spinal rod 12 relative to thebody of the screw.

The rod seat 18 may be configured to allow spinning of the spinal rod 12about the axis R-R. Alternatively, the second lower element 180 b and/orthe top element 181 may be configured to prevent rotation of the spinalrod 12 about the axis when the spinal rod 12 is seated in the rod seat.

After pivoting the spinal rod 12 about one or both selected axes in aselected direction relative to the bone anchor by a selected degree,preferably between 0° and 90° , a user can lock the orientation of therod 12 relative to the screw body 11 by inserting a closure mechanism,such as the set screw 170. The closure mechanism secures a spinal rod 12or other suitably configured spinal connection element within theU-shaped slot 142 of the head portion and locks the rod in the selectedorientation within and relative to the screw body. In the illustrativeembodiment, distal advancement of the closure mechanism into engagementwith the spinal rod 12 in the slot 142 seats the spinal rod in the seat18. Other suitable closure mechanisms may be employed to secure thespinal connection element to the assembly and/or to lock the orientationof the bone anchor relative to the receiving portion.

The first lower element 180 a and second lower element 180 b may bemovably retained in the head portion through any suitable means. Forexample, as shown in FIGS. 7-10, a retaining pin 191 may extend throughthe first lower element 180 a and second lower element 180 b and intothe body 11 of the screw to movably couple the lower portion 180 a, 180b of the rod seat 18 to the screw body 11. As shown in FIGS. 8A and 8B,the first lower element 180 a may include a slot 185 extendingsubstantially parallel to the axis R-R and perpendicular to the axis X-Xfor receiving the retaining pin 191. The retaining pin 191 slides withinthe slot during rotation of the first lower element within the recess144. The slot and pin facilitate rotation while preventing movement ofthe first lower element in other directions.

As shown in FIGS. 9A-9C, the second lower element 180 b includes alongitudinally extending hole 187 for receiving and seating thealignment pin 191. The hole 187 aligns with the axis X-X, such that thesecond lower element spins about the alignment pin 191. The hole 187 maycomprise a lower portion 187 a having a diameter corresponding to thediameter of the body 193 of the retaining pin 191, shown in FIG. 10. Thehole 187 includes an upper portion 187 for seating the head 195 of theretaining pin 191 to prevent the second lower element 180 b fromdecoupling.

According to another embodiment of the invention, the side surfaces1803, 1804 of the first lower element 180 a may have slots for slidablyreceiving protrusions formed in the side walls of the recess 144 toallow controlled rotation while retaining the first lower element 180 awithin the U-shaped slot 142. Alternatively, the side surfaces 1803,1804 may have protrusions that are slidably received in correspondingslots on the side walls of the recess 144.

In another embodiment, the outer bottom surface 1807 of theconical-shaped second lower element 180 b may include a horizontal slotor pin(s) configured to cooperate with a corresponding pin(s) or slot onthe upper surface 1802 of the first lower element 180 a. The cooperatingpin(s) and slot allow spinning of the second lower element 180 b withinthe first lower element 180 a.

One skilled in the art will recognize that any suitable means forcoupling the first and second lower elements 180 a and 180 b to eachother and/or to the screw body while allowing controlled relativeadjustment may be used.

According to another embodiment of the invention, shown in FIGS. 11 Aand 11B, the rod seat 18 of an adjustable bone screw assembly 10″ maycomprise a single lower element 180 for allowing controlled adjustmentof the spinal rod orientation relative to the body 11. The single lowerelement 180 defines both the surface 1801 that is received and movesrelative to the head portion 14, as well as the surface 1808 thatreceives the spinal rod. For example, the rod seat 18 may alternativelycomprise only the conical-shaped or other axisymmetric-shaped secondlower element 180 b disposed directly in a corresponding recess 144″ inthe slot 142 of the head portion 14 to allow rotation of the spinal rod12 received by the second lower element 180 b about only the X-X axisonly, as shown in FIGS. 11A and 11B.

Alternatively, the rod seat 18 may comprise only the cylindrical-shapedfirst lower element 180 a having a cylindrical lower surface 1801 forallowing pivoting about the Y-Y axis and an upper surface 1802 that isconfigured to receive the spinal rod 12 without a separately movableintermediate element (i.e., the second lower element 180 b).

The closure mechanism 170 may have any suitable size, shape,configuration and means for securing the rod and rod seat in a selectedorientation relative to the screw body 11. The closure mechanism 170 maybe secured to the screw body 11 through any suitable means, includingand exterior fixation device and an interior fixation devices. FIGS. 12Aand 12B illustrate two suitable means for securing a set screw 170against the rod seat 18 and spinal rod 12. For example, as shown in FIG.12A, an outside nut 171 may encircle the proximal end of the headportion 14 to secure the set screw 170 and lock the spinal rod 12 in aselected configuration relative to the screw body 11. According to analternate embodiment, shown in FIG. 12B, a typhoon cap 176 may bedisposed inside the head portion 14 to lock the set screw 170 inposition, thereby locking the orientation of the spinal rod 12 relativeto the screw body 11. One skilled in the art will recognize that anysuitable means for locking the spinal rod in a selected position andorientation relative to the screw body 11 may be used.

According to another embodiment, the top element 181 of the rod seat 18may be retained in the set screw 170 to avoid assembling multiple partsduring surgery.

The anchor portion 13 of the screw assembly can have any suitable size,configuration and shape and is not limited to a constant-diameter anchorportion as required in polyaxial screws of the prior art. According toone embodiment of the invention, shown in FIG. 13, the anchor portion13′ of the screw assembly 10 may be tapered to provide a larger diameternear the head portion 14. The use of a unitary screw body with anadjustable rod seat facilitates an anchor portion 13′ with a varyingdiameter. In contrast, in polyaxial screws of the prior art, the screwdiameter is limited by the diameter of the spherical top of the anchordisposed in the head portion. However, the biaxial screw assembly 10 ofthe illustrative embodiment of the invention places no limitation on theanchor diameter and shape.

In addition, the biaxial screw assembly 10 of the illustrativeembodiments of the invention allows for a reduction in the rod-to-bonedistance when the biaxial bone screw assembly 10 is employed to secure aspinal rod to bone. During use, a force is applied on a rod inserted inthe screw assembly at a distance D away from the bone surface (therod-to-bone distance). The force creates a moment on the bone screw,which is directly proportional to the distance D. The bone screwassembly 10 of the illustrative embodiments of the invention preferablyreduces the rod-to-bone distance, relative to bone screw assemblies ofthe prior art, thereby reducing the applied moment on the screw. Asmaller moment on the screw reduces the likelihood of screw-boneloosening and screw breakage.

For example, FIG. 14 illustrates a comparison between the rod-to-bonedistance D of the biaxial screw assembly 10 of the illustrativeembodiment of the invention compared with a rod-to-bone distance D1 of apolyaxial screw assembly 210 of the prior art and a rod-to-bone distanceD2 of a monoaxial screw assembly 220 of the prior art. For example, theillustrative biaxial bone screw assembly 10 has a rod-to-bone distance Dof between about 5 and 7 millimeters. As illustrated the rod-to-bonedistance D of the illustrative embodiment is about 5.58 millimeters. Therod-to-bone distance D1 of a monoaxial screw assembly 220 is alsobetween about 5 and 7 millimeters. As illustrated the rod-to-bonedistance of D1 of the illustrative embodiment is about 6.90 millimeters.In contrast, the rod-to-bone distance D2 of the polyaxial screw islarger, with distance between about 7 to 9 millimeters. As illustratedthe rod-to-bone distance of D2 of the illustrative embodiment is about8.77 millimeters. Hence, the biaxial screw assembly 10 provides asimilar rod-to-bone distance to that of monoaxial screw assembly 220,yet the biaxial screw assembly provides additional degrees-of-freedom toadjust the orientation of the spinal rod relative to the screw assembly.

FIG. 15 illustrates a comparison of the rod-to-bone distance of thebiaxial screw assembly of an illustrative embodiment of the invention,and a polyaxial screw of the prior art after rotation of the spinal rod12 by about 20 degrees. As shown, when the rod 13 inserted in apolyaxial screw head 214 is rotated 20 degrees, the moving head of thepolyaxial screw assembly 210 increases the rod-to-bone distance D1 to9.11 millimeters, while the biaxial screw assembly 10 of theillustrative embodiment of the invention retains a similar rod-to-bonedistance D of about 5.73 millimeters when the rod 12 is rotated 20degrees relative to the screw body 11, thereby minimizing breaks andloosening.

The illustrative adjustable bone screw assembly may be used tofacilitate connection between two vertebrae in the spinal column. Asurgeon may insert a spinal rod into a first bone screw assemblyanchored to a first vertebra, and then adjust the orientation of thespinal rod about one, two or more axes, as necessary, to align thespinal rod with a second bone screw assembly anchored to a secondvertebra. Another portion of the spinal rod may be inserted into thesecond bone screw assembly. The second bone screw assembly may alsopermit adjustment of the orientation of the spinal rod in one, two ormore selected directions, as necessary. After proper alignment, thespinal rods are locked in the selected positions relative to the bonescrew assemblies to connect the two vertebrae.

The components of the biaxial bone screw assembly of the illustrativeembodiments of the invention may be manufactured from any suitablebiocompatible material, including, but not limited to, metals and metalalloys such as titanium and stainless steel, polymers and/or ceramics.The components may be manufactured from the same or different materialsthough manufacturing processes known in the art.

The present invention has been described relative to an illustrativeembodiment. Since certain changes may be made in the above constructionswithout departing from the scope of the invention, it is intended thatall matter contained in the above description or shown in theaccompanying drawings be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention which, as a matter of language,might be said to fall therebetween.

1. A bone screw assembly comprising: a bone anchor having a distal shaftextending along a longitudinal axis configured to engage bone; aproximal head portion; and a rod seat disposed within the proximal headportion for seating the spinal rod, wherein the rod seat allows for acontrolled pivoting movement of the spinal rod about selected first andsecond axes in first and second directions relative to the head portion,wherein the pivoting movement of the spinal rod is limited aboutnon-selected axes, wherein the rod seat includes a first rotatable lowerelement and a second rotatable lower element stacked on the firstrotatable lower element, wherein the first rotatable lower element isconfined to rotate about the first axis in the first direction, and thesecond rotatable lower element is confined to rotate about the secondaxis in the second direction, wherein the second axis is different thanthe first axis.
 2. The bone screw assembly of claim 1, wherein theproximal head portion includes a U-shaped slot and a recess formed in abottom surface of the U-shaped slot for receiving the rod seat.
 3. Thebone screw assembly of claim 1, wherein the first lower element has abottom surface that is generated by sweeping a line segment about thefirst axis, thus allowing rotation about the first axis.
 4. The bonescrew assembly of claim 3, wherein the first lower element has acylindrical bottom surface that curves in the first direction.
 5. Thebone screw assembly of claim 1, wherein the second lower element has abottom surface that is axisymmetric about the second axis.
 6. The bonescrew assembly of claim 5, wherein the second lower element has aconical bottom surface.
 7. The bone screw assembly of claim 1, whereinthe first lower element has bottom surface that is axisymmetric aboutthe second axis and the second lower element has a bottom surfaceabutting a top surface of the first lower element, wherein the bottomsurface of the second lower element is generated by sweeping a linesegment about the first axis, thus allowing rotation about the firstaxis.
 8. The bone screw assembly of claim 1, wherein the first lowerelement includes a slot extending in the first direction and the secondlower element includes an opening aligned with the second axis and therod seat further comprises a retaining pin extending through the firstlower element, the second lower element and into the head portion tomovably couple the first lower element and the second lower element tothe head portion.
 9. The bone screw assembly of claim 1, wherein thesecond lower element includes an opening aligned with the second axisand the rod seat further comprises a retaining pin extending through thesecond lower element into the first element head portion to movablycouple the second lower to the first lower element.
 10. The bone screwassembly of claim 1, wherein the second lower element includes a recessfor receiving the spinal rod.
 11. The bone screw assembly of claim 1,wherein the rod seat further comprises a top element having a sphericaltop surface, and the spinal rod is received between a bottom surface ofthe top element and the second lower element.
 12. The bone screwassembly of claim 1, further comprising: a first upper element rotatableabout a longitudinal axis of the distal shaft of the proximal headportion; and a second upper element stacked between the first upperelement and the rod, the second upper element configured to enablepivoting of the rod seat.
 13. The bone screw assembly of claim 1,further comprising a pin for movably coupling the first rotatable lowerelement and the second rotatable lower element to the distal shaft. 14.The bone screw assembly of claim 1, wherein the head portion is fixedrelative to the anchor portion.
 15. The bone screw assembly of claim 1,wherein a rod-to-bone distance of the bone screw assembly is less thanabout six millimeters.
 16. The bone screw assembly of claim 1, whereinthe distal shaft of the anchor portion is tapered.
 17. The bone screwassembly of claim 1, wherein the rod seat pivots about a first axis thatis substantially perpendicular to a longitudinal axis of the rod seatand a longitudinal axis of the distal shaft and about a second axis thataligns with the longitudinal axis of the distal shaft.
 18. A bone anchorassembly comprising: a bone anchor having a distal shaft extending alonga longitudinal axis configured to engage bone and a proximal head,wherein the proximal head includes a first rotatable lower rod seatelement and a second rotatable lower rod seat element stacked on thefirst rotatable lower rod seat element; and a rod seat disposed in theproximal head for seating a spinal rod, wherein the rod seat includes atop surface for receiving the spinal rod and an axisymmetric bottomsurface that is symmetric about a longitudinal axis, the axisymmetricsurface received in a corresponding recess to allow pivoting of thespinal rod about first and second selected axes in first and seconddirections relative to the proximal head, wherein the pivoting of thespinal rod is limited about non-selected axes, wherein the firstrotatable lower rod seat element is confined to rotate about the firstaxis in the first direction, and the second rotatable lower rod seatelement is confined to rotate about the second axis in the seconddirection, wherein the second axis is different than the first axis. 19.The bone anchor assembly of claim 18, wherein the axisymmetric bottomsurface is conical-shaped.
 20. The bone anchor assembly of claim 18,wherein the corresponding recess is formed on the first rotatable lowerrod seat element, and the axisymmetric bottom surface is formed in thesecond rotatable lower rod seat element.
 21. The bone anchor assembly ofclaim 18, wherein the corresponding recess is formed in the proximalhead.
 22. The bone anchor assembly of claim 18, wherein: the firstrotatable lower rod seat element has a cylindrical bottom surface on afirst side disposed in a recess of the proximal head and defining thecorresponding recess on a second side; and the second rotatable lowerrod seat element defines the conical bottom surface on a first sidethereof that abuts the corresponding recess in the first rotatable lowerrod seat element and defines the top surface for receiving the spinalrod on a second side thereof.
 23. The bone anchor assembly of claim 22,wherein the cylindrical bottom surface of the first rotatable lower rodseat element curves in a selected direction to allow pivoting of thefirst rotatable lower rod seat element and the spinal rod in theselected direction relative to the bone anchor.
 24. A bone anchorassembly, comprising: a bone anchor portion having a distal shaftextending along a longitudinal axis configured to engage bone; and ahead portion fixed to a proximal end of the bone anchor portion, whereinthe head portion is configured to receive a spinal rod and allow for acontrolled pivoting movement of the spinal rod about first and secondselected axes in first and second directions relative to the headportion, wherein the pivoting movement of the spinal rod is limitedabout non-selected axes, wherein the rod seat comprises a firstrotatable lower rod seat element and a second rotatable lower rod seatelement stacked on the first lower rod seat element, wherein the firstrotatable lower rod seat element is confined to rotate about the firstaxis in the first direction, and the second rotatable lower rod seatelement is confined to rotate about the second axis in the seconddirection, wherein the second axis is different than the first axis. 25.The bone anchor assembly of claim 24, wherein the head portion includesa biaxially adjustable rod seat for seating the spinal rod.
 26. A methodof implanting a spinal rod, comprising the steps of: inserting a firstportion of the spinal rod in a first rod-receiving portion of a firstbone screw assembly connected to a first location, wherein the firstrod-receiving portion includes a rod seat comprising a first rotatableportion and a second rotatable portion stacked on the first rotatableportion; adjusting an orientation of the spinal rod relative to thefirst rod-receiving portion in a first direction; and adjusting anorientation of the spinal rod relative to the first rod-receivingportion in a second direction, wherein a movement of the spinal rod islimited about non-selected axes, wherein the first rotatable portion isconfined to rotate about a first axis in the first direction, and thesecond rotatable portion is confined to rotate about a second axis inthe second direction, wherein the second axis is different than thefirst axis.
 27. The method of claim 26, further comprising the step oflocking the spinal rod in a selected position relative to the firstrod-receiving portion.
 28. The method of claim 26, further comprisingthe steps of: inserting a second portion of the spinal rod in a secondrod-receiving portion of a second bone screw assembly connected to asecond location.
 29. The method of claim 26, further comprising the stepof locking the spinal rod in a selected position relative to the firstrod-receiving portion.